1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method using a terahertz wave, and more particularly to an apparatus and a method for visualizing or imaging information in a horizontal direction and a depth direction of an object based on a principle of an apparatus for measuring a terahertz wave in a time domain, i.e. terahertz time domain spectroscopy system (THz-TDS system).
2. Description of the Related Art
A terahertz wave is an electromagnetic wave having a component of a frequency band in a range of 0.03 THz or more and 30 THz or less. These frequencies contain many wavelengths which various substances such as biological molecules specifically absorb depending on their structure and condition. Taking advantage of these features, there has been developed an inspection technique for performing non-destructive material analysis and identification using a terahertz wave. Also, a terahertz wave is expected to be applied to a safe imaging technique as an alternative to an X-ray imaging technique as well as a high-speed communication technique.
In many cases, a terahertz wave having a shape of subpicosecond pulse is used for the above applications. In general, however, it is difficult to acquire such a pulse in real time. In light of this, a THz-TDS system performs sampling measurement by ultrashort pulse light (also referred to as excitation light in the present description) having a pulse width in the order of femtoseconds. This terahertz wave sampling is achieved by adjusting a time difference between a time when excitation light reaches a generation unit for generating a terahertz wave and a time when the excitation light reaches a detection unit for detecting the terahertz wave. For example, a stage having a folded optical system (also referred to as a delay optical unit in the present description) is inserted in a propagation path of an excitation light and an amount of folding of the excitation light is adjusted to acquire the time difference. In many cases, the generation unit or the detection unit is implemented by a photoconductive element including an antenna electrode pattern having a small gap formed on a semiconductor film.
When an object is imaged based on the principle of a THz-TDS system, a position of the terahertz wave irradiating the object is moved relatively in the surface direction with respect to the object. Imaging is attempted by measuring a time waveform of a terahertz wave for each pixel forming an image (see Japanese Patent No. 3387721). Such a system, however, needs to sequentially mechanically perform raster scanning on the places to be measured for each pixel, whereby it takes time to image the object. In order to solve this problem, there has been disclosed a method of collectively acquiring the time waveform of a plurality of pixels using one photoconductive element (see Appl. Phys. Lett. 93, 121105 (2008)). In this method, an object is irradiated with a terahertz wave of a substantially collimated form and a terahertz wave from the object is subjected to spatial modulation by a metal opening pattern corresponding to a pixel. When the metal opening is opened, the terahertz wave is transmitted therethrough, but when the metal opening is closed, the terahertz wave is reflected thereby, whereby a spatial distribution depending on the presence or absence of a terahertz wave changes according to the opening pattern. Thus, in an imaging step, a change in this metal opening pattern is monitored to extract each pixel signal based on the spatial distribution of the terahertz wave.